The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys

Abstract

Zr-Nb alloys are known to perform better in corrosion and hydrogen pick-up than other Zr alloys but the mechanism by which this happens is not well understood. Atomistic simulations using density functional theory of both tetragonal and monoclinic ZrO2 were performed, with intrinsic defects and Nb dopants. The overall defect populations with respect to oxygen partial pressure were calculated and presented in the form of Brouwer diagrams. Nb is found to favour 5 + in monoclinic ZrO2 at all partial pressures, but can exist in oxidation states ranging from 5 + to 3 + in the tetragonal phase. Nb5+ is charge balanced by Zr vacancies in both phases, suggesting that contrary to previous assumptions, Nb does not act as an n-type dopant in the oxide layer. Clusters containing oxygen vacancies were considered, Nb2+ was shown to exist in the tetragonal phase with a binding energy of 2.4 eV. This supports the proposed mechanism whereby low oxidation state Nb ions (2 + or 3+) charge balance the build-up of positive space-charge in the oxide layer, increasing oxygen vacancy and electron mobility, leading to near-parabolic corrosion kinetics and a reduced hydrogen pick-up. Previous experimental work has shown that tetragonal ZrO2 transforms to the monoclinic phase during transition, and that during transition a sharp drop in the instantaneous hydrogen pick-up fraction occurs. The oxidation of lower charge state Nb defects to Nb5+ during this phase change, and the consequent temporary n-doping of the oxide layer, is proposed as an explanation for the drop in hydrogen pick-up during transition

Hydrogen pickup during oxidation in aqueous environments: The role of nano-pores and nano-pipes in zirconium oxide films

“…Given these rate-limiting factors, one could rationalize the effect of alloying Zr with Nb which empirically is known to reduce the hydrogen pickup fraction [67][68][69]. Comparing Figure 2(a) and (c), one can see that the oxide films in both of the Zr-Nb samples, Figure 2(a) and (b) have a lower concentration of nano-porosity than the oxide on Zircaloy-4, Figure 2(c), which would correlate directly with the measured lower hydrogen pick up fraction because fewer H atoms could reach the metal-oxide boundary.…”

Investigating the stability of second phase particles in Zr-Nb alloys under irradiation

“…This oxidation process commonly consists of pre-transition and transition cycles [3], and while the oxide film grows slowly in the pre-transition regime, obeying 'parabolic' or 'sub-parabolic' growth rate laws [4] [5], 'break-away' growth occurs in the transition regime with much higher oxidation rates [3]. The onset of the detrimental 'break-away' phenomenon of accelerated oxide film growth has been shown to be related to micro-structural and micro-chemical properties of the metal and oxide [6], including the choice of major alloying elements like Nb or Sn [7][8] [9] and the size and chemistry of second phase particles (SPPs) [10]. Under inreactor conditions, the harsh working environment can also affect the corrosion resistance.…”

Modelling and experimental analysis of the effect of solute iron in thermally grown Zircaloy-4 oxides

“…The effects of iron additions on the types of defects and their concentrations in ZrO 2 are investigated by carrying out density functional theory (DFT) simulations of iron defects in isolation and in clusters with other iron and intrinsic defects. The need to consider the effect of bound defects on oxidation state, defect populations and therefore overall oxide behaviour was shown by Bell et al for both tin and niobium [38,39]. It was shown for tin that including paired defects significantly increased the oxygen partial pressure at which the 2+ state was stable [39].…”